This invention relates to a material for reinforcing a panel such as an automobile door panel or roof panel.
A conventional reinforcing structure is shown in FIGS. 1(A), 1(B), and 1(C). A reinforcing member 4 is made of a thermosetting resin augmented with a reinforcing material composed of glass fibers or non-woven fabric. The reinforcing member 4 is affixed to the inner surface of a door outer panel 2 (metallic plate-like member 2a) of a door 1. Thereafter, the reinforcing member 4 is hardened by heating.
In FIG. 1, 5 is the door inner panel, 6 is the door window sash, 7 is the side window glass and 8 is the outside door handle.
In the case of such a reinforcing structure, a significant reinforcing effect cannot be obtained unless a large quantity of the resin material is used, which would increase the production cost and increase the weight. Also, it is difficult to conform the reinforcing member precisely to the shape of the panel to be reinforced, if the reinforcing member is molded and hardened in advance of its application to the plate-like member.
If a reinforcing structure is formed by bonding a reinforcing member consisting of unhardened resin to the panel and thermosetting the same, still the thick resin layer will not conform to areas of the panel with a small radius of curvature.
In either case, complete adhesion of the reinforcing member to the panel is difficult to achieve.
With these problems in mind, we previously proposed a panel for an automobile. As illustrated in FIGS. 2(A) and 2(B), a thermosetting resinous reinforcing member 11 is made of a tough epoxy resin, for example, augmented with a reinforcing material 10 such as glass fibers. The reinforcing member 11 is applied directly onto the inner surface 2a of a metallic panel 2 serving as a door outer panel, or on the coating film 9 plated onto said inner surface in advance for rust proofing.
A reinforcing rib 13 is integrally formed on the inner surface of the panel 2. The reinforcing rib 13 is composed of the thermosetting-resinous reinforcing member 11 and a foamable portion.
As shown in FIG. 3, a reinforcing material 14 is used to form the reinforcing rib 13. The reinforcing material is made by laminating an unhardened resin material 16 such as an unhardened epoxy resin over an expandable or foamable material 15 which has not yet expanded. Particularly the expandable material 15 is preferably a gas cell-forming material which will form a foam after heating, such as a strip of polyethylene foam or a foamable epoxy strip in the pre-foam state. By heating the expandable strip to expand the same, for example, during the coating-drying step at a later stage of automotive assembly, a bead-like portion 12 will be formed.
In order for the reinforcing rib 13 to have the desired height and strength, the expandable or foamable material 15 must expand in such a manner that the projection achieves a predetermined height. Generally, the thickness of the foamable material 15 should be reasonably large.
For this reason, even if the reinforcing material is formed in a sheet, the expandable material 15 will have some noticeable rigidity. The reinforcing material will not securely adhere to the panel 2 if the surface 2a of the panel 2 is irregular because the resin material will not conform completely to the complicated shape of the panel 2. Some portions of the resin material are apt not to adhere to the panel so that some small spaces will be present between the panel and the resin material. When the foamable material swells, such small spaces will increase in volume. As a result, the reinforcing effect will decrease.